Project Summary/Abstract In the retina, visual information is quickly segregated into pathways that respond to increases and decreases in light intensity. At the first retinal synapse, the tonic release of glutamate from photoreceptor terminals maintains a high synaptic concentration in darkness that rapidly decreases in response to light. Two types of postsynaptic cells, the ON- and OFF-bipolar cells (BPCs), respond with opposite polarity to glutamate released by photoreceptors, thus establishing the opposing visual pathways that are maintained throughout the rest of the visual system. The basis of signaling in OFF-BPCs, which relies on the activation of ionotropic glutamate receptors, is well understood; the signaling pathway that generates the light response in ON-BPCs, however, is more complex, and the molecular mechanisms remain to be elucidated. The ON-BPC signaling pathway originates with a unique metabotropic glutamate receptor, mGluR6, which is found exclusively on the dendrites of ON-BPCs. mGluR6 acts via a G-protein, GO, to regulate the activity of an unidentified cation channel such that the light-induced decrease in glutamate opens the channel and depolarizes the cell. In many ways, this sequence of events resembles the well-studied signal transduction pathway of photoreceptor outer segments, in which photoexcitation of rhodopsin is coupled via the G-protein, transducin, to the closure of a cGMP-gated cation channel. In the outer segment, the kinetics of the light response is largely determined by the lifetime of activated transducin. Deactivation of transducin occurs upon hydrolysis of GTP by the transducin alpha subunit, and this reaction is accelerated by interaction with the G5-RGS9-R9AP complex. Mutations in the gene encoding any one of these three proteins severely impair vision by slowing recovery after light flashes. We have identified two similar complexes, G5-RGS7 and G5-RGS11, in ON-BPC dendrites suggesting a similar mechanism of deactivation of the ON-BPC signal transduction pathway. We hypothesize that the RGS- G5 complexes are critical components of the mGluR6 signal transduction pathway in ON-BPC dendrites. Using a combination of biochemical, immunohistochemical, and electrophysiological approaches, we will test this hypothesis by answering the following questions: 1. How do RGS-G5 complexes shape the response of ON-BPCs to light? 2. How are these RGS complexes anchored in the ON-BPC dendrites and how does this affect their function? 3. What other proteins in the mGluR6 pathway interact with G5-RGS7 and G5-RGS11? The data from this study will contribute to the elucidation of the signaling pathway in the ON-bipolar cell, a fundamental, yet poorly understood, step in visual processing.